Chryses, a priest of Apollo and father of Chryseis, came to Agamemnon to ask for the return of his daughter. Agamemnon refused, and insulted Chryses, who prayed to Apollo to avenge his ill-treatment. Enraged, Apollo afflicted the Achaean army with plague. Agamemnon was forced to return Chryseis to end the plague, and took Achilles' concubine Briseis as his own. Enraged at the dishonour Agamemnon had inflicted upon him, Achilles decided he would no longer fight. He asked his mother, Thetis, to intercede with Zeus, who agreed to give the Trojans success in the absence of Achilles, the best warrior of the Achaeans.